Remove _PyUnicode_AsKind from private C API

[serhiy-storchaka]

BPO	40130
Nosy	@vstinner, @serhiy-storchaka
PRs	bpo-40130: _PyUnicode_AsKind() should not be exported. #19265

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-04-01.12:42:11.740>
created_at = <Date 2020-03-31.21:15:43.464>
labels = ['expert-C-API', 'type-feature', '3.9']
title = 'Remove _PyUnicode_AsKind from private C API'
updated_at = <Date 2020-04-01.12:42:11.739>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-04-01.12:42:11.739>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-01.12:42:11.740>
closer = 'serhiy.storchaka'
components = ['C API']
creation = <Date 2020-03-31.21:15:43.464>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40130
keywords = ['patch']
message_count = 2.0
messages = ['365427', '365459']
nosy_count = 2.0
nosy_names = ['vstinner', 'serhiy.storchaka']
pr_nums = ['19265']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40130'
versions = ['Python 3.9']

[serhiy-storchaka]

_PyUnicode_AsKind is exposed as private C API. It is only used in unicodeobject.c, where it is defined. Its name starts with an underscore, it is not documented and not included in PC/python3.def (therefore is not exported on Windows). Seems it is not used in third party code (I have not found any occurrences on GitHub except CPython clones).

Initially it was also used in Python/formatter_unicode.c, and I think it is the only reason of exposing it in the header. I think that now it can be removed.

The proposed PR removes _PyUnicode_AsKind from headers, makes it static, rename it, and change its signature (since all the kind, data pointer and length are available at the caller site). It also includes minor cleanup and microoptimizations.

[serhiy-storchaka]

New changeset 17b4733 by Serhiy Storchaka in branch 'master':
bpo-40130: _PyUnicode_AsKind() should not be exported. (GH-19265)
17b4733